The present invention relates generally to thermal energy management systems and cooling circuits within electron beam generating devices and systems. More particularly, the present invention relates to a system for controlling coolant volume size within an x-ray tube.
A computed tomography (CT) imaging system typically includes a gantry that rotates at various speeds in order to create a 360° image. The gantry contains a CT tube, which generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential is created across the vacuum gap, which allows electrons to be emitted, in the form of an electron beam. The electron beam is emitted from the cathode to a target on the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target, where they are abruptly slowed down to emit x-rays. The high voltage potential produces a large amount of heat within the CT tube, especially within the anode.
The vacuum vessel is typically enclosed in a casing filled with circulating cooling fluid, such as dielectric oil. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and the cathode. The cooling fluid in cooling the vacuum vessel maintains temperatures thereof and components contained therein. The temperature maintenance of the CT tube aids in the prevention of image artifacts, as well as increasing the life of the CT tube components.
The cooling fluid within the CT tube typically has a high coefficient of thermal expansion (CTE). In other words, the cooling fluid volume of the fluid can increase and decrease significantly with change in temperature. Currently a moveable diaphragm is used to compensate for the expansion of the cooling fluid. For CT imaging systems that use a separate heat exchanger for the CT tube, during a CT tube maintenance replacement, the cooling fluid volume can become maladjusted when the CT tube and corresponding cooling circuit is at an elevated temperature.
When a CT tube is replaced, the replacement CT tube and the cooling fluid contained therein are at room temperature. The CT tube being replaced is typically at a temperature above room temperature. Although, the volume of the cooling fluid within the replacement CT tube is approximately the same as the volume of the cooling fluid within the CT tube being replaced, the actual amount of room temperature fluid in the replacement tube is greater than that of the CT tube being replaced. Thus, the replacement in effect increases the amount of fluid within the cooling circuit. This increase in the amount of fluid can be as much as one third of a liter, which upon heating of the replacement CT tube can result in the fluid volume expanding beyond a mechanical limit of the diaphragm. The expansion beyond the mechanical limit creates an overpressure situation within the cooling circuit. This overpressure situation can cause the cooling circuit to operate inappropriately and eventually cause the system to become inoperable.
Thus, there exists a need for a CT tube cooling circuit or associated system that is capable of accounting for a change in cooling fluid volume upon replacement or maintenance of a CT tube.